1. Field of the Invention
The present invention relates generally to a numerical control screw tightening machine, and particularly concerns the numerical control screw tightening machine capable of automatically detecting screw holes of a work by utilizing an ultrasonic wave and can automatically control the position of an electric driver, thereby enabling automatic and precise tightening of a screw.
2. Description of the Prior Art
As shown in FIG. 1, a conventional numerical control screw tightening machine comprises a numerical control apparatus 1 and a screw tightening apparatus 2. The numeric control apparatus 1 comprises a central processing unit 3, a program memory means 4, a first numerical control means 6 for driving a movable table 9, etc. and the second numerical control means 7 for driving an electrical screw driver 11 and stopping the electrical screw driver 11, etc. after a stopping of the table movement. The screw tightening apparatus 2 comprises a table driving means 8, X-Y table 9, a head member 10, an electrical screw driver 11 and a screw supplying source 12. The conventional numerical control screw tightening machine tightens screws into screw holes of an object work, which are disposed at a predetermined position on a conveyer line and have a predetermined relative positional relation against the screw tightening apparatus 2, according to a predetermined screw tightening data. In the above-mentioned conventional machine, the object work should be disposed precisely at the predetermined position of the conveyer line for preventing a defective screw tightening. Therefore, a roller conveyer can not be used but a module type conveyer which can precisely dispose the holes should be used. But the machine using the module type conveyer has a disadvantage that it is expensive. Further, when various kinds of works are served for screw tightening, exclusive palettes designed for each kind of work are usually made and the work is disposed in the palette and the palette is supplied by the module type conveyer. Therefore, in this case, there is also a disadvantage that the cost of making the palettes is expensive. Furthermore, concerning with a big scale work, on which the processing errors of the screw hole position and pitch are liable to be of an order of about 2 mm, even though the work is fixed precisely on the palette, the relative positional relation between the hole of the work and the screw tightening apparatus 2 is liable to get out of order largely. Therefore, the screw tightening according to the predetermined screw tightening data can not be executed precisely and automatic producting system including screw tightening process have not been realized.
For dissolving the above-mentioned disadvantages, hitherto another screw tightening machine using an ITV (industrial television) has been offered. The machine is such that an image of the work including a hole image is optically produced by an ITV camera, and a central position of the hole is detected by processing the image. However, the machine function is liable to be effected by a color or contrast of the work or a brightness of a circumference of a working place, and the machine is expensive. Therefore, the machine is not adequate against a screw tightening machine.